DMARC Working Group K. Andersen
Internet-Draft LinkedIn
Intended status: Experimental B. Long, Ed.
Expires: July 26, 2018 Google
S. Jones, Ed.
TDP
S. Blank, Ed.
ValiMail
M. Kucherawy, Ed.
TDP
January 22, 2018
Authenticated Received Chain (ARC) Protocoldraft-ietf-dmarc-arc-protocol-11
Abstract
The Authenticated Received Chain (ARC) protocol creates a mechanism
whereby a series of handlers of an email message can conduct
authentication of the email message as it passes among them on the
way to its destination, and record the status of that authentication
at each step along the handling path, for use by the final recipient
in making choices about the disposition of the message. Changes in
the message that might break DKIM or DMARC can be identified through
the ARC set of header fields.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at https://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on July 26, 2018.
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Internet-Draft ARC-Protocol January 2018B.1.3. Example 1: Message received by Recipient . . . . . . 37B.2. Example 2: Mailing list to forwarded mailbox . . . . . . 38B.2.1. Here's the message as it exits the Origin: . . . . . 38B.2.2. Message is then received at example.org . . . . . . . 39B.2.3. Example 2: Message received by Recipient . . . . . . 43
B.3. Example 3: Mailing list to forwarded mailbox with source 45
B.3.1. Here's the message as it exits the Origin: . . . . . 45B.3.2. Message is then received at example.org . . . . . . . 46B.3.3. Example 3: Message received by Recipient . . . . . . 51Appendix C. Acknowledgements . . . . . . . . . . . . . . . . . . 53Appendix D. Comments and Feedback . . . . . . . . . . . . . . . 54
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 541. Introduction
Modern email authentication techniques such as the Sender Policy
Framework (SPF) [RFC7208] and DomainKeys Identified Mail (DKIM)
[RFC6376] have become common.
However, their end-to-end utility is limited by the effects of
intermediaries along the transmission path, which either are not
listed (for SPF) or which break digital signatures (for DKIM). These
issues are described in substantial detail in those protocols'
defining documents as well as in [RFC6377] and [RFC7960].
Technologies that build upon the use of SPF and DKIM can reduce the
success of fraudulent email campaigns. To this end, Domain-based
Mail Authentication, Reporting and Compliance (DMARC) [RFC7489],
validates the domain of the RFC5322.From author header field.
However its use along email transmission paths that have independent
intermediaries, such as some forwarders and essentially all mailing
list services, produces false positive rejections that are
problematic, both for the message authors, the intermediary
service(s), and for those they are interacting with.
What is needed is a mechanism by which legitimate alteration of a
message, which invalidates associated SPF and DKIM information, does
not ultimately result in a rejection of an email message on delivery.
Authenticated Receive Chain (ARC) builds upon DKIM mechanisms to
provide a sequence of signatures that are more survivable than DKIM's
and that provide a view of the handling sequence for a message,
especially the points where alterations of the content might have
occurred. Equipped with this more complete information, the
recipient system(s) can make a more informed handling choice,
reducing or eliminating the false negatives inherent in use of DKIM
and/or SPF themselves.
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Internet-Draft ARC-Protocol January 20182. Overview
In DKIM, every participating signing agent attaches a signature that
is based on the some of the content of the message, local policy, and
the domain name of the participating Administrative Management Domain
(ADMD). Any verifier can process such a signature; a verified
signature means that the domain referenced in the DKIM-Signture's
"d=" parameter has some responsibility for handling the message. An
artifact of using digital signature technology for this means that
verification also ensures that the message content that was "covered"
by the signature has not been altered since the signature was
applied. The signatures themselves are generally independent of one
another.
By contrast, an ARC signature conveys the following pieces of
information:
1. An assertion that, at the time that the intermediary ADMD
processed the message, the various assertions (DKIM-Signature(s)
and/or ARC sets) already attached to the message by other ADMDs
were or were not valid;
2. As with DKIM, an assertion that, for a validated signature, the
domain name in the signature takes some responsibility for
handling of the message and that the message is unchanged since
that signature was applied;
3. A further assertion that binds the ARC evaluation results into
the ARC chain sequence.
This protocol accomplishes each of these by adding a new header field
to the message for each of these pieces of information, as follows:
o ARC-Authentication-Results (referred to below as "AAR"): virtually
identical in syntax to an Authentication-Results field [RFC7601],
this field records the results of all message authentication
checks done by the recording ADMD at the time the message arrived.
Additional information is placed in this field compared to a
standard Authentication-Results field in order to support a more
complete DMARC report (see Section 5.2);
o ARC-Message-Signature (referred to below as "AMS"): virtually
identical in syntax to DKIM-Signature, this field contains the
signature about the message header and body as they existed at the
time of handling by the ADMD adding it; and
o ARC-Seal (referred to below as "AS"): highly similar in structure
and format to a DKIM-Signature, this field applies a digital
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Internet-Draft ARC-Protocol January 2018
signature that protects the integrity of all three of these new
fields when they are added by an ADMD, plus all instances of these
fields added by prior ADMDs.
A distinguishing feature of all of these is that an ARC participant
always adds all of them before relaying a message to the next
handling agent en route to its destination. Moreover, as described
in Section 5.1, they each have an "instance" number that increases
with each ADMD in the handling chain so that their original order can
be preserved and the three related header fields can be processed as
a group.
3. Definitions and Terminology
This section defines terms used in the rest of the document.
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
Because many of the core concepts and definitions are found in
[RFC5598], readers SHOULD to be familiar with the contents of
[RFC5598], and in particular, the potential roles of intermediaries
in the delivery of email.
Syntax descriptions use Augmented BNF (ABNF) [RFC5234].
o "ARC set" - A single group of the header fields introduced in
Section 2 is called an "ARC set".
o "ARC chain" - The complete sequence of these groups (ARC sets) is
called an "Authenticated Received Chain" or merely an "ARC chain".
Although the "Received" header field is typically not included in
the signed content, the name is based on the notion that this is
in essence a cryptographically signed series of header fields that
attest to the handling chain of a message much as Received fields
always have.
3.1. Referenced Definitions
The following terms are defined in other RFCs. Those definitions can
be found as follows:
o ADMD - [RFC5598], Section 2.3
o MTA - [RFC5598], Section 4.3.2
o MSA - [RFC5598], Section 4.3.1Andersen, et al. Expires July 26, 2018 [Page 6]

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o MDA - [RFC5598], Section 4.3.3
The three header fields that are part of this specification borrow
heavily from existing specifications. Rather than repeating all of
the formal definitions that are being reused in ARC, this document
only describes and specifies changes in syntax and semantics.
Language, syntax, and other details are imported from DKIM [RFC6376].
Specific references can be found below.
4. Protocol Elements and Features
As with other domain authentication technologies (such as SPF, DKIM,
and DMARC), ARC makes no claims about the contents of the email
message it has sealed. However, for a valid and passing ARC chain, a
Final Receiver is able to ascertain:
o all (participating) domains that claim responsibility for handling
(and possibly) modifying the email message in transit;
o trace information, including:
* the [RFC7601] authentication results each participating ADMD
saw; and
* additional data needed to compile a DMARC report for the
sending domain.
Given this information, a Final Receiver is able to make a more-
informed local policy decision regarding message delivery to the end
user in spite of a DMARC failure.
Every participant in an ARC chain, except for the originating sender
and Final Receiver, is both an ARC Validator (when receiving) and
then an ARC Sealer (when sending a message onward). The validated
chain status as determined at message receipt must be passed to the
sealer function in order for sealing to occur properly; how this is
done is considered ADMD-specific and an implementation detail.
_INFORMATIONAL_: It is important to understand that validating and
then immediately sealing a message leaves no room for message
modification, and many early implementations of ARC did not initially
work because both operations were performed in a single pass over the
message.
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Internet-Draft ARC-Protocol January 20184.1. Features of the ARC Protocol
The following protocol features are functional parts and design
decisions related the protocol that are not specific to either
Validators or Sealers, but ensure the ARC chain conveys this
information to a Final Receiver.
4.1.1. Chain of Custody
At a high level, an ARC chain represents a chain of custody of
authentication and other trace information (AAR) related to a
message, signed by each handler of the message. Each link in the
chain (AMS) is designed to be brittle, insofar as it survives only
until the next modification of the message. However, the sequence of
intermediaries in the handling chain (AS) is designed to remain
intact over the entirety of the chain.
The ARC chain can be conceptualized through an analogy with the chain
of custody for legal evidence. The material evidence itself is
sealed within an tamper-proof bag (AMS) each time. When handed to a
new party, that party both vouches for the state of the received
evidence container (AAR) and signs for the evidence on a chain of
custody report form (AS). As with all analogies, this one should not
be taken to interpretive extremes, but primarily used as a conceptual
framework.
An ARC chain that is valid and passing has the attributes listed
above in Section 4.
Recipients of an ARC chain that is invalid or does not pass SHOULD
NOT draw negative conclusions without a good understanding of the
wider handling context. Until ARC usage is widespread,
intermediaries will continue to modify messages without ARC seals.
As with a failing DKIM signature ([RFC6376] Section-6.3), a failing
ARC chain SHOULD be treated the same as a message with no ARC chain.
[[ NOTE TO WORKING GROUP: This paragraph probably is better placed in
Verifier actions. ]]
4.1.2. Optional Participation
Validating an existing chain and then adding your own ARC set to a
message allows you to claim responsibility for hanndling the message
and modifications, if any, done by your ADMD to benefit message
delivery downstream. However, no ADMD is obligated to perform these
actions.
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Internet-Draft ARC-Protocol January 20184.1.3. Only one ARC Chain (One Chain to Rule Them All)
A message can have only one ARC chain on it at a time (see
Section 5.1). Once broken, the chain cannot be restarted, as the
chain of custody is no longer valid and responsibility for the
message has been lost.
4.1.4. All Failures are Permanent
Because ARC chains are transmitted across multiple intermediaries,
all errors, even temporary ones, become unrecoverable and are
considered permanent.
Any error validating or sealing a chain, for whatever reason, MUST
result in a "cv=fail" verdict.
4.1.5. Benign nature of an ARC Set
Even when an ARC chain is valid and passes, its value is limited to
very specific cases. An ARC chain is specifically designed to
provide value to a Final Receiver evaluating message delivery in the
context of a DMARC failure. An ARC chain in general, and each ARC
set in particular, provide additional information, and otherwise is
benign. Specifically:
o properly adding an ARC set to a message does not damage or
invalidate an existing chain, and
o validating a message exposes no new threat vectors (see
Section 13).
_INFORMATIONAL_: If an ADMD is unsure whether it will be re-emitting
and/or modifying a message, it may elect to seal all inbound mail.
For complex or nested ADMD relationships such as found in some hosted
mail solutions, this "inbound seal" can be used to facilitate
traversal of internal boundaries as well as properly conveying
incoming state to any egress MTAs that may need to assert a seal upon
exit from the ADMD. Since these internal relationships are highly
implementation dependent, this protocol definition can not usefully
explore such usage except to note that it is intentionally allowed
within the scope of this specification.
4.1.6. Key Management
The public keys for ARC header fields follow the same requirements,
syntax and semantics as those for DKIM signatures, described in
Section 3.6 of [RFC6376]. ARC places no requirements on the
selectors and/or domains used for the ARC header field signatures.
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Internet-Draft ARC-Protocol January 20184.1.7. Trace Information
ARC includes trace information encoded in the AAR. While section
Section 5.2 defines what information must be provided, sealing ADMDs
may provide additional information, and validating receivers may use
or ignore this trace information as they wish.
4.1.8. Instance Tags
See section Section 5.14.1.9. Chain Validation Status
See section Section 5.4.15. The ARC Header Fields5.1. Instance ('i=') Tag
The header fields comprising a single ARC set are identified by the
presence of a string in the value portion of the header field that
complies with the "tag-spec" ABNF found in Section 3.2 of [RFC6376].
The tag-name is "i" and the value is the text representation of a
positive integer, indicating the position in the ARC sequence this
set occupies, where the first ARC set is numbered 1. In ABNF terms:
instance = [FWS] %x69 [FWS] "=" [FWS] position [FWS] ";"
position = 1*2DIGIT ; 1 - 50
Valid ARC sets must have exactly one instance of each header field
for a given position value and signing algorithm. (Initial
development of ARC is only being done with a single allowed signing
algorithm, but parallel work in the DCRUP working group
(https://datatracker.ietf.org/wg/dcrup/about/) is expanding that.
For handling multiple signing algorithms, see [ARC-MULTI].)
Because the AMS and AS header field values are made up of tag-spec
constructs, the i= tag may be found anywhere within the header field
value, but is represented throughout this spec in the initial
position for convenience. Implementers are encouraged to place the
i= tag at the beginning of the field value to facilitate human
inspection of the headers.
5.1.1. Valid Range for Instance Tags
The 'i' tag value can range from 1-50 (inclusive).
ARC implementations MUST support at least ten (10) ARC sets.
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An effective operational maximum will have to be developed through
deployment experience in the field and will be documented within
[ARC-USAGE] once determined.
ARC chains with more than the defined operational maximum count MUST
be marked with "cv=fail".
5.2. ARC-Authentication-Results (AAR)
The ARC-Authentication-Results header field is syntactically and
semantically identical to an Authentication-Results header field
(defined in Section 2.2 of [RFC7601] (A-R)), except that several
optional data fields SHOULD be added. ([[ NOTE: these optional data
fields are being proposed as amendments to [RFC7601] through a "bis"
process. Depending on the sequencing for this specification and said
"7601bis" work, it may be possible to drop the noted sections from
this specification. ]]) The first element ("Authentication-Results:")
in authres-header is replaced with arc-authres-prefix as follows:
arc-authres-header-prefix = "ARC-Authentication-Results:" [CFWS] arc-info
arc-info = instance *([CFWS] propspec) [CFWS] ";"
The purpose of this header field is to transmit the results of any
authentication done on the message upstream to participating ADMDs
validating and continuing the chain.
The AAR MUST contain all A-R results from within the participating
ADMD, regardless of how many A-R headers are on the message.
5.2.1. ptypes and properties for arc-info
[[ NOTE: This section is being proposed as an amendment to [RFC7601]
through a "bis" process. Depending on the sequencing for this
specification and said "7601bis" work, it may be possible to this
section from this specification. ]]
Certain information pertinent to ascertaining message disposition can
be lost in transit when messages are handled by intermediaries. For
example, failing DKIM signatures are sometimes removed by MTAs, and
most DKIM signatures on messages modified by intermediaries will
fail.
The AAR, through these ptype-properties stamped in arc-info, provide
a mechanism for this information to survive transit.
The ptypes and properties defined in this section SHOULD be stamped
in the AAR:
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o smtp.client-ip - The connecting client IP address from which the
message is received;
o header.s - Defined in [RFC6376] section 7.2
o arc.oldest-pass - The instance number of the oldest AMS that still
validates, or 0 if all pass.
5.3. ARC-Message-Signature (AMS)
The ARC-Message-Signature header field is syntactically and
semantically identical to a DKIM-Signature header field [RFC6376],
with the following exceptions:
o There is an "i" tag, as described in Section 5.1.
o There is no "v" tag.
ARC-Seal header fields MUST NOT be included in the content covered by
the signature in this header field.
The AMS SHOULD include any DKIM-Signature header fields already
present on the message in the header fields covered by this
signature.
The AMS header field MAY include (sign) the AAR header field(s).
Authentication-Results header fields SHOULD NOT be included since
they are likely to be deleted by downstream ADMDs (per Section XXX of
[RFC7601]), thereby breaking the AMS signature.
As with a DKIM-Signature, the purpose of this header field is to
allow the ADMD generating it to take some responsibility for handling
this message as it progresses toward delivery.
5.4. ARC-Seal (AS)
The ARC-Seal header field is syntactically and semantically similar
to a DKIM-Signature field, with the following exceptions:
o There is an "i" tag, as described in Section 5.1.
o The ARC-Seal covers none of the body content of the message. It
only covers specific header fields. (See below: Section 5.4.2.)
As a result, no body canonicalization is done. Further, only
"relaxed" header canonicalization (Section 3.4.2 of [RFC6376]) is
used.
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o The only supported tags are "i" (Section 5.1 supercedes the
[RFC6376] definition), and "a", "b", "d, "s", "t". The latter 5
tag definitions are copied from Section 3.5 of [RFC6376].
o An additional tag, "cv" is defined. (See below: Section 5.4.1)
5.4.1. The 'cv' Tag
A new tag "cv" (chain validation) indicates the the outcome of
evaluating the existing ARC chain upon arrival at the ADMD that is
adding this header field. It accepts one of three possible values:
o none: There was no chain on the message when it arrived for
validation; typically occurs when the message arrives at a Message
Transfer Agent (MTA) from a Message Submission Agent (MSA) or when
any upstream MTAs may not be participating in ARC handling;
o fail: The message has a chain whose validation failed;
o pass: The message has a chain whose validation succeeded.
In ABNF terms:
seal-cv-tag = %x63.76 [FWS] "=" [FWS] ("none" / "fail" / "pass")
5.4.2. Implicit Header Fields
The ARC-Seal signs a canonicalized form of the ARC set header values.
The ARC set header values are compiled in increasing instance order,
starting at 1, and inclue the set being added at the time of sealing
the message.
Within a set, the header fields are listed in the following order:
1. ARC-Authentication-Results
2. ARC-Message-Signature
3. ARC-Seal
Where the ARC-Seal is the one being generated, it is input to the
hash function in its final form except with an empty "b=" value, in
the same manner by which a DKIM-Signature signs itself.
Note that the signing scope for the ARC-Seal is modified in the
situation where a chain has failed validation (see Section 7.1).
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Internet-Draft ARC-Protocol January 20186. Verifier Actions
A verifier takes the following steps to determine the state of the
ARC chain on a message (cv value). Canonicalization, hash functions,
and signature validation methods are imported from Section 5 of
[RFC6376].
[[ Note: need markdown flag to have subordinate numbering distinction
]]
1. Collect all ARC sets currently on the message. If there were
none, the ARC state is "none" and the algorithm stops here.
2. If the form of any ARC set is invalid (e.g., does not contain
exactly one of each of the three ARC-specific header fields),
then the chain state is "fail" and the algorithm stops here.
1. To avoid the overhead of unnecessary computation and delay
from crypto and DNS operations, the cv value for all ARC-
Seal(s) MAY be checked at this point. If any of the values
are "fail", then the overall state of the chain is "fail" and
the algorithm stops here.
3. Conduct verification of the ARC-Message-Signature header field
bearing the highest instance number. If this verification fails,
then the chain state is "fail" and the algorithm stops here.
4. For each ARC-Seal from the "N"th instance to the first, apply the
following logic:
1. If the value of the "cv" tag on that seal is "fail", the
chain state is "fail" and the algorithm stops here. (This
step SHOULD be skipped if the earlier step (2.1) was
performed)
2. In Boolean nomenclature: if ((i == 1 && cv != "none") or (cv
== "none" && i != 1)) then the chain state is "fail" and the
algorithm stops here (note that the ordering of the logic is
structured for short-circuit evaluation).
3. Initialize a hash function corresponding to the "a" tag of
the ARC-Seal.
4. Compute the canonicalized form of the ARC header fields, in
the order described in Section 5.4.2, using the "relaxed"
header canonicalization defined in Section 3.4.2 of
[RFC6376]. Pass the canonicalized result to the hash
function.
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5. Retrieve the final digest from the hash function.
6. Retrieve the public key identified by the "s" and "d" tags in
the ARC-Seal, as described in Section 4.1.6.
7. Determine whether the signature portion ("b" tag) of the ARC-
Seal and the digest computed above are valid according to the
public key. (See also Section Section 6.1 for failure case
handling)
8. If the signature is not valid, the chain state is "fail" and
the algorithm stops here.
5. If all seals pass validation, then the chain state is "pass", and
the algorithm is complete.
[[ Note from Dave: possibly delete the following paragraph as it is
more usage/procedural than specification guidance.
KA: It was added to clarify the separation of the verification and
signing steps as some of the initial implementations failed to
realize that they were not necessarily done in one fell swoop.
KA (v-10): With the addition of the {protocol-elements} section, does
the WG think that this can be reasonably removed from this location?
]]
The verifier should save the cv state for subsequent use by any
sealing which may be done later (potentially after message
modification) within the same trust boundary. The cv state may be
recorded by sealing at the time of verification in an initial ARC set
(for the ADMD) or may be recorded out of band depending on the
architecture of the ADMD.
6.1. Handling DNS Problems While Validating ARC
DNS-based failures to verify a chain are treated no differently than
any other ARC violation. They result in a "cv=fail" verdict.
6.2. Responding to ARC Validity Violations During the SMTP Transaction
If a receiver determines that the ARC chain has failed, the receiver
MAY signal the breakage through the extended SMTP response code 5.7.7
[RFC3463] "message integrity failure" [ENHANCED-STATUS] and
corresponding SMTP response code.
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Internet-Draft ARC-Protocol January 20187. Signer Actions
[[ Note from Dave: This seems more like implementation guidance than
specification detail. KA: see explanation just above referring to
the previous note. ]]
This section includes a specification of the actions an ARC signer
takes when presented with a message.
The signer MUST undertake the following steps:
1. Before creating an ARC signature, perform any other, normal
authentication and/or signing, so that the ARC signature can
cover those results.
2. Build and attach the new ARC set:
1. If an ARC chain exists on the message, then set "N" equal to
the highest instance number found on the chain (i=);
otherwise set "N" equal to zero for the following steps.
2. Generate and attach to the message an ARC-Authentication-
Results header field using instance number N+1 and the same
content from the previous step.
3. Generate and attach to the message an ARC-Message-Signature
header field as defined in Section 5.3 above, using instance
number N+1.
4. Generate and attach to the message an ARC-Seal header field
using the general algorithm described in Section 5.4 above,
the chain validation status as determined in Section 6, and
instance number N+1.
7.1. Marking and Sealing "cv=fail" (Invalid) Chains
The header fields signed by the AS header field b= value in the case
of a chain failure MUST be only the matching 'i=' instance headers
created by the MTA which detected the malformed chain, as if this
newest ARC set was the only set present.
8. Usage of ARC and Chain Validity8.1. Relationship between DKIM-Signature and AMS signing scopes
[[ SB-11: replace with "ARC MUST be the last signer of the message;
otherwise it cannot be validated on receipt." in the signer actions
section. KA: Concern that this still does not address the risk of
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DKIM-Signatures covering ARC chains. This does not seem like it fits
in this section but it needs to go somewhere. ]]
DKIM-Signatures SHOULD never sign any ARC header fields.
[[ KA: Response to Dave's concern: If DKIM covers ARC and ARC covers
DKIM, which comes first? The chicken or the egg? I'm open to
alternate ways to phrase this without opening the "modifying the DKIM
spec" can of worms. ]]
8.2. Assessing Chain Validity Violations
[[ SB-11: move to protocol elements
KA: This seems a bit banal. I suspect that what receivers want is a
section on using ARC to override DMARC failures. Would that be to
brazen to insert here instead? Or do we relegate that to the
[ARC-USAGE] doc? ]]
Email transit can produce broken signatures for a wide variety of
benign reasons. This includes possibly breaking one or more ARC
signatures. Therefore, receivers need to be wary of ascribing motive
to such breakage although patterns of common behaviour may provide
some basis for adjusting local policy decisions.
ARC does not attempt to protect an entire message. There are various
ways that a message can still be problematic, in spite of having a
valid ARC chain. Consequently, all normal, content-based analysis
SHOULD still be performed on any message having a valid chain of ARC
header sets.
9. Recording and Reporting the Results of ARC Evaluation
The evaluation of an ARC chain provides information which will be
useful to both the receiver (or intermediary) and to the initial
sender of the message. This information should be preserved and
reported as follows.
9.1. Information from an ARC Evaluation
The evaluation of an ARC chain produces a list of domain names for
participating intermediaries which handled the message, to wit:
o A list of the "d=" domains found in the validated ARC-Seal header
fields
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o The "d=" domain found in the most recent (highest instance number)
AMS header field (since that is the only one necessarily
validated)
In the case of a failed chain, only the terminal ARC set is covered
by the ARC-Seal so the reporting is limited to the findings in that
terminal ARC set.
9.2. Recording (local) ARC Evaluation Results
Receivers MAY add an "arc=[pass|fail|policy]" method annotation into
a locally-affixed Authentication-Results [RFC7601] header field along
with any salient comment(s).
Details of the ARC chain which was evaluated should be included in
the Authentication-Results and AAR headers per Section Section 5.2.1.
9.3. DMARC Reporting of ARC Findings - Interim
[[ Note: Discussion on the IETF DMARC-WG list has indicated some
interest in more substantial reporting for analytic purposes. To
support that effort, the following guidance is provided only as an
interim, minimal data set. A more complete reporting construct will
be specified in a related spec - TBD. (see the additional fields
specified in Section 5.2.1) ]]
Receivers SHOULD indicate situations in which ARC evaluation
influenced the results of their local policy determination. DMARC
reporting of ARC-informed decisions can be accomplished by adding a
local_policy comment explanation containing the list of data
discovered in the ARC evaluation (Section 9.1 and Section 5.2.1):
<policy_evaluated>
<disposition>delivered</disposition>
<dkim>fail</dkim>
<spf>fail <comment>source.ip=10.0.0.1</comment></spf>
<reason>
<type>local_policy</type>
<comment>arc=pass ams[2].d=d2.example ams[2].s=s1 as[2].d=d2.example
as[2].s=s2 as[1].d=d1.example as[1].s=s3</comment>
</reason>
</policy_evaluated>
In the suggested sample, d2.example is the sealing domain for ARC[2]
and d1.example is the sealing domain for ARC[1].
Mediators SHOULD generate DMARC reports on messages which transit
their system just like any other message which they receive. This
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o Header field name: ARC-Message-Signature
Applicable protocol: mail
Status: draft
Author/Change controller: IETF
Specification document(s): [I-D.ARC]
Related information: [RFC6376]
o Header field name: ARC-Authentication-Results
Applicable protocol: mail
Status: standard
Author/Change controller: IETF
Specification document(s): [I-D.ARC]
Related information: [RFC7601]
13. Security Considerations
The Security Considerations of [RFC6376] and [RFC7601] apply directly
to this specification.
13.1. Header Size
Inclusion of ARC sets in the header of emails may cause problems for
some older or more constrained MTAs if they are unable to accept the
greater size of the header.
13.2. DNS Operations
Operators who receive a message bearing N ARC sets have to complete
up to N+1 DNS queries to evaluate the chain (barring DNS redirection
mechanisms which can increase the lookups for a given target value).
This has at least two effects:
1. An attacker can send a message to an ARC partipant with a
concocted sequence of ARC sets bearing the domains of intended
victims, and all of them will be queried by the participant until
a failure is discovered. The difficulty of forging the signature
values should limit the extent of this load to domains under
control of the attacker.
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2. DKIM only does one DNS check per signature, while this one can do
many (per chain). Absent caching, slow DNS responses can cause
SMTP timeouts; and backlogged delivery queues on mediating
systems. This could be exploited as a DoS attack.
13.3. Message Content Suspicion
Recipients are cautioned to treat messages bearing ARC sets with the
same suspicion that they apply to all other email messages. This
includes appropriate content scanning and other checks for
potentially malicious content. The handlers which are identified
within the ARC chain may be used to provide input to local policy
engines in cases where DMARC validation fails (due to mediation
impacting SPF attribution, DKIM validity or alignment).
Note that a passing ARC chain may not adequately mean that the
message is safe because:
1. You have to trust all signatories; and
2. Even trusted systems may have become compromised or may not
properly authenticate messages, so even with a chain of trusted
participants, the message might still never have authenticated in
the first place (which is why you have the AAR to inspect) or
could have been subject to unintended modifications.
14. Evaluating the Efficacy of the ARC Protocol
The ARC protocol is designed to mitigate some of the most common
failure conditions for email which transits intermediary handlers en
route to the final recipient. Some of these problems have happened
due to the adoption of the DMARC protocol [RFC7489] and are listed in
[RFC6377] and [RFC7960].
As the ARC protocol becomes standardized and implemented amongst
intermediary handlers, the following aspects should be evaluated in
order to determine the success of the protocol in accomplishing the
intended benefits.
NOTE: Terminology within this section does NOT follow [RFC2119]
interpretation. This section represents the current thoughts of the
working group regarding unanswered questions related to the protocol.
Wider deployment will inform these topics and probably expand them.
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Internet-Draft ARC-Protocol January 201814.1. Success Consideration
Currently, many receivers have heuristically determined overrides in
order to rescue mail from intermediary-caused failures. Many of
those overrides rely on inferrence rather than direct evidence.
ARC will be a success if, for ARC sealed messages, receivers are able
to implment ARC-based algorithmic decisions based on the direct
evidence found within the ARC chain. This is especially relevant for
DMARC processing when the DKIM d= value is aligned with the
rfc5322.From author domain.
14.2. Failure Considerations
The intent of ARC is to be at most value-add and at worst benign. If
ARC opens up significant new vectors for abuse (see Section 13) then
this protocol will be a failure. Note that weaknesses inherent in
the mail protocols ARC is built upon (such as DKIM replay attacks and
other known issues) are not new vectors which can be attributed to
this specification.
14.3. Open Questions
The following open questions are academic and have no clear answer at
the time of the development of the protocol. However, wide-spread
deployment should be able to gather the necessary data to answer some
or all of them.
14.3.1. Value of the ARC-Seal (AS) Header
Data should be collected to show if the ARC-Seal (AS) provides value
beyond the ARC Message Signature (AMS) for either making delivery
decisions or catching malicious actors trying to craft or replay
malicious chains.
14.3.2. DNS Overhead
Longer ARC chains will require more queries to retrieve the keys for
validating the chain. While this is not believed to be a security
issue (see Section 13.2), it is unclear how much overhead will truly
be added. This is similar to some of the initial processing and
query load concerns which were debated at the time of the DKIM
specification development.
Data should be collected to better understand usable length and
distribution of lengths found in valid ARC chains along with the the
DNS impact of processing ARC chains.
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Internet-Draft ARC-Protocol January 201814.3.3. Distinguishing Valuable from Worthless Trace Information
There are several edge cases where the information in the AAR can
make the difference between message delivery or rejection. For
example, if there is a well known mailing list that ARC seals but
doesn't do its own initial DMARC enforcement, a Final Receiver with
this knowledge could make a delivery decision based upon the
authentication information it sees in the corresponding AAR header.
Certain trace information in the AAR is useful/necessary in the
construction of DMARC reports. It would be beneficial to identify
the value-add of having intermediary-handled mail flow information
added into the DMARC reports going back to senders.
Certain receivers believe the entire set of trace information would
be valuable to feed into machine learning systems to identify fraud
and/or provide other signals related to message delivery.
It is unclear what trace information will be valuable for all
receivers, regardless of size.
Data should be collected on what trace information receivers are
using that provides useful signals that affect deliverability, and
what portions of the trace data are left untouched or provide no
useful information.
Since many such systems are intentionly proprietary or confidential
to prevent gaming by abusers, it may not be viable to reliably answer
this particular question. The evolving nature of attacks can also
shift the landscape of "useful" information over time.
15. Implementation Status
[[ Note: For minimizing section number references when the RFC editor
removes this section, it has been moved to be the last section of the
document before the Appendicies. ]]
[[ Note to the RFC Editor: Please remove this section before
publication along with the reference to [RFC6982]. ]]
This section records the status of known implementations of the
protocol defined by this specification at the time of posting of this
Internet-Draft, and is based on a proposal described in [RFC6982].
The description of implementations in this section is intended to
assist the IETF in its decision processes in progressing drafts to
RFCs. Please note that the listing of any individual implementation
here does not imply endorsement by the IETF. Furthermore, no effort
has been spent to verify the information presented here that was
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supplied by IETF contributors. This is not intended as, and must not
be construed to be, a catalog of available implementations or their
features. Readers are advised to note that other implementations may
exist.
This information is known to be correct as of the seventh
interoperability test event which was held on 2017-07-15 & 16 at
IETF99.
For a few of the implementations, later status information was
available as of December 2017.
15.1. GMail test reflector and incoming validation
Organization: Google
Description: Internal production implementation with both debug
analysis and validating + sealing pass-through function
Status of Operation: Production - Incoming Validation
Coverage: Full spec implemented as of [ARC-DRAFT-06]
Licensing: Proprietary - Internal only
Implementation Notes:
o Full functionality was demonstrated during the interop testing on
2017-07-15.
Contact Info: arc-discuss@dmarc.org [1]
15.2. AOL test reflector and internal tagging
Organization: AOL
Description: Internal prototype implementation with both debug
analysis and validating + sealing pass-through function
Status of Operation: Beta
Coverage: ARC chain validity status checking is operational, but only
applied to email addresses enrolled in the test program. This system
conforms to [ARC-DRAFT-06]
Licensing: Proprietary - Internal only
Implementation Notes:
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o 2017-07-15: Full functionality verified during the interop
testing.
Contact Info: arc-discuss@dmarc.org [2]
15.3. dkimpy
Organization: dkimpy developers/Scott Kitterman
Description: Python DKIM package
Status of Operation: Production
Coverage:
o 2017-07-15: The internal test suite is incomplete, but the command
line developmental version of validator was demonstrated to
interoperate with the Google and AOL implementations during the
interop on 2017-07-15 and the released version passes the tests in
[ARC-TEST] (https://github.com/ValiMail/arc_test_suite) with both
python and python3.
Licensing: Open/Other (same as dkimpy package = BCD version 2)
Contact Info: https://launchpad.net/dkimpy15.4. OpenARC
Organization: TDP/Murray Kucherawy
Description: Implemention of milter functionality related to the
OpenDKIM and OpenDMARC packages
Status of Operation: Beta
Coverage: Built to support [ARC-DRAFT-10]
Licensing: Open/Other (same as OpenDKIM and OpenDMARC packages)
Implementation Notes:
o The build is FreeBSD oriented but some packages have been built
for easier deployment on RedHat-based Linux platforms.
o Some issues still exist when deploying in a chained milter
arrangement (such as OpenSPF -> OpenDKIM -> OpenDMARC -> OpenARC)
with coordination between the stages. When deployed in a
"sandwich" configuration around an MLM, there is no effective
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mechanism to convey trust from the ingress (validator) to egress
(signer) instances. (_NOTE_: this is expected to resolved with a
new release of OpenDMARC expected in January 2018.)
Contact Info: arc-discuss@dmarc.org [3]
15.5. Mailman 3.2 patch
Organization: Mailman development team
Description: Integrated ARC capabilities within the Mailman 3.2
package
Status of Operation: Patch submitted
Coverage: Based on OpenARC
Licensing: Same as mailman package - GPL
Implementation Notes:
o Appears to work properly in at least one beta deployment, but
waiting on acceptance of the pull request into the mainline of
mailman development
Contact Info: https://www.gnu.org/software/mailman/contact.html15.6. Copernica/MailerQ web-based validation
Organization: Copernica
Description: Web-based validation of ARC-signed messages
Status of Operation: Beta
Coverage: Built to support [ARC-DRAFT-05]
Licensing: On-line usage only
Implementation Notes:
o Released 2016-10-24
o Requires full message content to be pasted into a web form found
at http://arc.mailerq.com/ (warning - https is not supported).
o An additional instance of an ARC signature can be added if one is
willing to paste a private key into an unsecured web form.
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Internet-Draft ARC-Protocol January 201815.9. PERL Mail::Milter::Authentication module
Organization: FastMail
Description: Email domain authentication milter, uses MAIL::DKIM (see
above)
Status of Operation: Intial validation completed during IETF99
hackathon with some follow-on work during the week
Coverage: Built to support [I-D.ARC]
Licensing: Open Source
Implementation Notes:
o 2017-07-15: Validation functionality which interoperates with
Gmail, AOL, dkimpy was demonstrated; later in the week of IETF99,
the signing functionality was reported to be working
o 2017-07-20: ARC functionality has not yet been pushed back to the
github repo but should be showing up soon
Contact Info: https://github.com/fastmail/authentication_milter15.10. Sympa List Manager
Organization: Sympa Dev Community
Description: Work in progress
Status of Operation: Work in progress
Coverage: unknown
Licensing: open source
Implementation Notes:
o 2018-01-05: Tracked as https://github.com/sympa-community/sympa/issues/153
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Internet-Draft ARC-Protocol January 2018Appendix A. Appendix A - Design Requirements
(This section is re-inserted for background information from
[ARC-DRAFT-06] and earlier versions.)
The specification of the ARC framework is driven by the following
high-level goals, security considerations, and practical operational
requirements.
A.1. Primary Design Criteria
o Provide a verifiable "chain of custody" for email messages;
o Not require changes for originators of email;
o Support the verification of the ARC header field set by each hop
in the handling chain;
o Work at Internet scale; and
o Provide a trustable mechanism for the communication of
Authentication-Results across trust boundaries.
A.2. Out of Scope
ARC is not a trust framework. Users of the ARC header fields are
cautioned against making unsubstantiated conclusions when
encountering a "broken" ARC sequence.
Appendix B. Appendix B - Example Usage
[[ Note: The following examples were mocked up early in the
definition process for the spec. They no longer reflect the current
definition and need various updates which will be included in a
future draft. ]]
(Obsolete but retained for illustrative purposes)
B.1. Example 1: Simple mailing listB.1.1. Here's the message as it exits the Origin:Andersen, et al. Expires July 26, 2018 [Page 34]